Patellar arthroplasty may be performed to treat cartilage damage, arthritis, or injury to the patellofemoral joint. In a patellar arthroplasty, a portion of the patella is replaced with a patellar implant or prosthesis. The prosthesis may have a posterior-facing bearing surface for articulation with the natural medial and lateral condyles situated on the distal end of the femur, or for articulation with a femoral implant. An anterior surface of the implant anchors to the remaining natural or prepared patella.
Patellar implants known in the art include onlay and inset style implants. Patella reaming guides are used presently in patellar resurfacing procedures for both onlay and inset style patella implants. For onlay implants, an oversized reamer collet is used to surround the entire exposed posterior surface of the patella and resurface the complete posterior patella with one reaming step. For inset implants, a slightly smaller reamer collet is used to ream only a portion of the patella. Onlay implants may be oval shaped to approximate the exposed cut area. Alternatively, some onlay implants are smaller than the exposed area and leave a portion of the cut bone exposed, which may create problems with soft tissue ingrowth. Furthermore, onlay patellar implants may be more prone to migrating or loosening then inset implants. The reaming step used to prepare the patella for the onlay implant also does little to conserve the amount of bone volume remaining in the patella, which can lead to complications such as patellar fracture. While an inset “button” implant may completely cover the resurfaced portion of the bone, the smaller prosthesis area may not sufficiently remove all areas of diseased cartilage and the patient may be left with pathology. Need exists for patellar implants and bone preparation instrumentation which conserve bone volume while also allowing for removal of all areas of diseased cartilage.
Patellar implants known in the art include a single flat, or uni-planar, anterior surface for anchoring to patellar bone. A single flat anchoring surface may not provide resistance to shear loads, and may therefore be more prone to loosening and migration. Need exists for implants with geometry that leverages two or more non-parallel surfaces at the bone-contacting interface, in order to provide better resistance to shear loads and be more resistant to loosening and migration. Having two or more planes at the bone-contacting interface allows the patella to be prepared to better match the native patella geometry, and may contribute to patellar bone conservation. In addition, better overall coverage of the articular surface may be provided by an implant having two or more planes at the bone-contacting interface. Instrumentation and methods for preparing a multi-planar prepared bone surface for receiving an implant with a multi-planar anchoring surface are also needed.